For example, in the case of a rod-shaped sushi (bo-zushi), vinegared rice is shaped, and the shaped rice section is topped with a sushi material or materials such as boned and prepared red sea bream, sockeye salmon or mackerel together with a thin slice of kombu kelp (shiroita kombu) cooked in a broth seasoned with vinegar, sugar and salt, and resultant is shaped into various shapes such as the rod-shape, and the shaped sushi product is wrapped with a bamboo sheath and placed in a packing bag made of a plastic film and vacuum-packed, the vacuum packed sushi product is frozen with a brine such as ethanol or the (vacuum-packed) shaped sushi product is frozen by slow freezing such as air-blast freezing or nitrogen or carbon dioxide freezing, and the frozen shaped sushi product is placed in a packing bag made of a plastic film and vacuum-packed to thereby prepare a vacuum-packed frozen sushi product.
Since such packed frozen sushi products frozen prepared using brine freezing process are vacuum-packed before the freezing, the attachment of bacteria and absorption of odor from the coolant gas in a freezing room, and therefore, these are hygienically superior to, for example, packed frozen sushi products prepared by vacuum-packing sushi products posterior to freezing of the sushi products by slow freezing such as air-blast freezing, nitrogen or carbon dioxide freezing. In particular, such packed frozen sushi products frozen by brine freezing can be frozen-stored stably, for example, over a long period of 3 months or longer when stored at a storage temperature of −18° C., or can be stored for 6 months when stored at a storage temperature of −50° C.
However, packed frozen sushi products which have been frozen-stored need to be thawed before being served at a meal. Methods of thawing for packed frozen sushi products include natural thawing, running water thawing, steam thawing, hot water thawing and microwave thawing. Of these, microwave thawing is quite suitable for thawing of packed frozen sushi products due to its extremely short thawing time and high thawing yield. For example, in the case of above mentioned rod-shaped sushi product, natural thawing at room temperature will require five hours, while microwave thawing will allow thawing in four minutes, enabling instant and convenient response to the demands compared to natural thawing at room temperature.
However, in the case of microwave thawing, there are differences in the quantitative value and the calorific value from microwave absorption between the sushi material or materials and the rice, and furthermore, because the surface area is better heated compared to the center area, a part of the sushi material or materials is heated to above the preferred temperatures compared to the rice, where the sushi material or materials result in the so-called cooked condition and is considered to impair the flavor, therefore, this method of thawing is in actuality not in use. Furthermore, in the case of a rod-shaped sushi for example, the rice section is commonly preferred to be effected to a tepid temperature, for example, between 15 to 25° C., preferably between 20 to 25° C., however, the sushi material or materials section is preferred to be effected to temperatures lower than the rice section, for example between 10 to 25° C., preferably between 10 to 25° C. Meanwhile, when thawing a packed frozen sushi product using a microwave oven, the material or materials are heated to relatively high temperatures so to impair the flavor of the sushi, making thawing using a microwave oven with the desired result difficult. Given this factor, in order to overcome the hurdle of thawing a packed frozen sushi product using electromagnetic heating such as one using a microwave oven, where the sushi flavor is impaired by the sushi material or materials getting heated to relatively high temperatures, the present inventors suggested the so-called packed frozen sushi product, in which a sushi topped with its sushi material or materials is packed in a plastic packing film with a metal foil positioned over the plastic film to cover the sushi material or materials on top of the rice, packing it further with a plastic packing film, placing this packed sushi product in a plastic packing bag, degassing and hermetically packing before the entire product is wholly brine frozen in the above mentioned condition for frozen storage (see Japanese Patent publication No. 3201933, Japanese Patent Publication No. 3229766 and Japanese Patent Publication No. 3369550).
In the case of such packed frozen sushi products, where a plastic packing film is placed over the sushi material or materials, with a metal foil placed over it, and the above mentioned rice and the sushi material or materials are packed together in a plastic packing film, when thawing using the electromagnetic heating process such as in a microwave oven, the rice section left uncovered and exposed to microwave irradiation is heated by the microwave, however, the sushi material or materials section shielded from the microwave by the metal foil is left unheated by the microwave, and remains halfway thawed essentially. However, heat applied to the rice will generate water vapor from the rice section, and the water vapor generated will fill the space inside the plastic packing film, and steam heat the space inside the plastic packing film during heating inside a microwave oven and after removing the sushi from the microwave oven, enabling the rice to warm to a tepid temperature and the sushi material or materials to warm to approximately 10° C.
As such, a packed frozen sushi product, packed in a plastic packing film with a metal foil positioned over the sushi material or materials to prevent the material or materials from being heated by the microwave, is steamed by the water vapor generated at the time of thawing, and therefore is suitable for thawing using a microwave oven. However, a packed frozen sushi product with a metal foil placed over it and packed in a plastic packing film is required to place the metal foil on top of the sushi material or materials packed inside the first plastic packing film before packing the sushi with the second plastic packing film, in order to electrically isolate the metal foil from the surroundings so to prevent any discharge from the sides of the metal foil at the time of microwave irradiation. Furthermore, the metal foil needs to be placed over the material or materials of the sushi without getting misaligned, when packing the sushi with the second plastic packing film, and also, the sushi placed on the plastic packing film needs to be packed air-tight. These procedures required in manufacturing packed frozen sushi products packed in plastic packing films require much attention and skill, and are considered to be an obstacle to mass manufacturing. Packed frozen sushi products require much manpower and time, and remain as a problem in order to respond to the customer demands.
It is an object of the present invention to provide a packed sushi product which is capable of solving the problems inherent in the frozen sushi product wrapped with a plastic film for wrapping and which is adapted for thawing by means of a microwave oven.